A luminescent material sensor of existing temperature measuring systems is pulsed with excitation radiation, and the rate of decay of luminescence after termination of the excitation pulse is measured. This decay time is then correlated to the temperature or other parameter being measured by the luminescent material sensor. Although other temperature dependent characteristics of a wide variety of optical sensors have been suggested or incorporated into commercial temperature measuring systems, the luminescent decay time approach appears to be the most accepted.
With this approach, the luminescent material may be attached to a surface of an object whose temperature is being measured, or may be included as part of a small sensor at an end of a length of optical fiber. In the surface coating case, excitation radiation is directed onto the surface and the resulting luminescence is collected from the luminescent material layer without contacting it. In the fiberoptic sensor case, the excitation radiation is sent along the length of optical fiber from its free end to the sensor and the resulting luminescence is communicated back along the fiber from the sensor to its free end. An electro-optical system is connected to the free end of the fiber in order to provide the excitation radiation pulses and to detect and process the luminescence emission signal.
Early commercial fiberoptic sensor products using luminescence decay time technology have found their greatest use in research and development laboratories and for scientific applications. A significant advantage of a fiberoptic temperature sensor is that it is non-electrically conductive and thus can be used in environments having strong electrical or magnetic fields that precludes the use of metallic temperature sensors such as thermocouples. In order to provide high accuracy and repeatability of measurements, the instruments of these systems are rather complex and expensive.
Therefore, there has been effort in recent years to produce a practical fiberoptic temperature sensing system that is much less complex, and thus which can be manufactured and sold at a lower cost in order to compete more favorably with other forms of temperature measurement systems. In 1991, Luxtron Corporation, assignee of the present application, brought to market a much simplified product that has most of the necessary optics and electronics mounted on a single printed circuit card or board for measuring the temperature dependent decay time of an attached fiberoptic luminescent material sensor. This card can be included within larger systems where temperature measurements are required to be made for controlling operation of the systems or simply for informational purposes. A electronic signal output of the card provides the luminescent signal decay time which is then converted to temperature by an output module, a dedicated host computer or an existing computer of a larger system into which the card is installed. Alternatively, for other applications, the card can be provided with the processing necessary to output a signal giving temperature of the sensor directly. This commercial board product of Luxtron Corporation is described in its U.S. Pat. No. 5,107,445. The commercial board product of Luxtron Corporation has been using the chromium activated yttrium gallium garnet ("Cr:YGG") luminescent material described in this patent.
This board product, much simpler, smaller and less expensive than prior measuring instruments, was found to have some limitations. It is a primary object of the present invention to improve the accuracy and stability of various parameter measuring systems of which this board product is an example.
It is another object of the present invention to provide a design technique and resulting measuring system that may be implemented at a lower cost.
It is a further object of the present invention to improve the yield of accurate measurement boards during their manufacture.